The present invention relates to the field of ultrasonic defect detecting systems, and especially to such systems which are used for nondestructive inspection (NDI) of elements having varying thicknesses. This invention has particular application in the testing of aircraft structures made from graphite/epoxy materials.
There are three major types of NDI systems which are used for testing elements, for example, aircraft parts: loss-of-back (LOB), pulse echo (PE) and through transmission ultrasonic (TTU). The LOB technique compares a predetermined threshold value with the peak amplitude of the ultrasonic reflections from an element's rear surface, i.e., the surface most distant from the ultrasonic transducers. If the element has no defects in the volume between the front and back surfaces proximate the transducers, then the peak amplitude of the reflections from the back surface should exceed the threshold. If a defect is present in that volume, the peak amplitude of the signal reflected by the rear surface decreases significantly, in fact below the threshold, because the defect reflects much of the ultrasonic energy before it ever reaches the rear surface.
FIGS. 1A and 1B, which show voltage signals corresponding to ultrasonic reflections from an element having no defects and having a defect, respectively, illustrate this phenomenon. There are three major reflection portions in FIG. 1A. The portion that occurs first, which is the leftmost in FIG. 1A, is an artifact from the ultrasonic pulse transmitted toward the element (sometimes called the "Main Bang"). The next major portion is a reflection from the front surface of the element. The third major portion (the rightmost) is a reflection from the rear surface. Since FIG. 1A corresponds to the reflections received from an element with no defects, the peak amplitude of the reflections from the rear surface is above the predetermined threshold V.sub.N, thereby indicating the absence of any defects.
In FIG. 1B, there are four major signal portions proceeding in order from left to right corresponding in time to their receipt by an ultrasonic transducer. The first, and largest, is the artifact from the Main Bang, the second represents a reflection from the front surface of a part, the third represents a reflection from a defect in the interior of the part, and the fourth represents a reflection from the rear surface of the part. Because the defect reflects some of the ultrasonic energy that penetrates the front surface, a smaller amount of energy is available to be reflected from the rear surface. As FIG. 1B shows, that rear surface reflection is below the predetermined threshold V.sub.N, so the presence of a defect is noted.
FIGS. 1A and 1B also show the concept of a time window which is used for finding the proper signals for testing. The time period denoted T.sub.R indicates a time window during which reflections or transmitted signals from the rear surface are expected to be received. It is important in the LOB technique to know when that window should begin and end to ensure that the reflections being examined are those from the rear surface, and not those from a defect or the front surface.
The PE technique bears some similarity to the LOB technique. However, instead of examining the peak amplitude of the rear surface reflection as the LOB technique requires, the PE techniques tests the peak amplitudes of the reflections from the element's interior, i.e., from between the front and rear surfaces. If any reflections from the interior above a certain threshold level are received, those reflections are evidence of a defect. If no sufficiently large reflections are received from the element interior, then the element portion under investigation is deemed defect-free.
The TTU technique differs from the above techniques in that it requires two transducers for each transducer channel, the transducers being located on opposite sides of the element to be examined. Instead of examining ultrasonic reflections, however, the TTU technique involves determining the amount of ultrasonic energy that was able to pass entirely through the part.
As the above methods indicate, it is extremely important to control the time window in which the examination takes place. For example, in the LOB method, the time window must be such as to capture only reflections from the rear surface (see T.sub.R in FIGS. 1A and 1B). In the PE method, it is extremely important to obtain a time window that captures reflections between the front and rear surfaces, and which does not include reflections from either of those surfaces (see time window T.sub.I in FIGS. 1A and 1B).
It is not difficult to identify the reflection from the front surface because that is the first reflection that occurs after the Main Bang artifact. If the element being examined has a varying thickness, however, it becomes very difficult to determine where the rear surface is because the location of that surface changes in relation to the front surface, and hence the corresponding time windows related to the rear surface must also change. Furthermore, the only ultrasonic information which is available to find the rear surface are the reflections from the part under test. However, as FIGS. 1A and 1B show, the reflections from a defect and from a rear surface appear very similar. Furthermore, by the time a reflection is properly identified, the ultrasonic detector is usually making another measurement.
One solution to this problem has been to determine thickness mechanically with a calibrated roller, for example. Rollers, however, react slowly and are inaccurate not only because they may lose contact with the surface, but also because the rollers experience wear which gradually makes their measurements imprecise. In addition, rollers cannot be used in many instances. For example, an element may be mounted or configured in a manner to preclude the use of a roller, or the temperature of the elements, for example molten steel sheets, may be too extreme for rollers.
Another way of solving the problem was discussed in U.S. Pat. No. 3,942,358 to Pies. The device in this patent includes an array of transducers which both transmit and receive ultrasonic pulses in the PE mode. The transducers are coupled to electric circuitry which measures the time difference between receipt of the surface reflection and the next major reflection, and then finds the maximum time difference. That maximum time difference is stored and compared to the maximum speed elapsed times determined from succeeding scans. Whenever a maximum time measurement from a succeeding scan exceeds the stored amount, the new maximum time is stored in place of the old value. The result of the entire operation is that the maximum time difference for the entire element, and hence the maximum thickness, is stored and used to set a time window corresponding to the rear surface.
In this system, however, two ultrasonic scans need to be made for each element. The first scan determines the maximum thickness, and the second scan then looks for defects. In addition, since each transducer channel is used for determining thickness during one scan and defects during the next scan, the electronics of this system can become rather complex.
Pies does recognize that for elements of varying thickness, the transmit times could be updated during the scans. This still results in complicated circuitry, however, to perform the thickness measurement task. The system in Pies also cannot detect extensive defects.
It is therefore an object of this invention to provide fast and accurate NDI ultrasonic testing of parts.
Another object of this invention is accurate NDI testing without the use of complicated circuitry.